Filter pad

ABSTRACT

Filter pads, which are made inherently stable by binders and by removal of the binders can be converted in packings to be maintained under substantially constant surface pressure between rigid filter brackets, are produced first as molded bodies from a suspension of filter aids in liquid by depositing granular, fibrous, or fiber-containing filter aids or mixtures of those filter aids in a casting mold, there being used high rates of flow while avoiding turbulences, and adding binders to the suspension so that the molded body can be stabilized by activation of the binder while drying. It is possible in this process also to manufacture multi-layered filter pads, it having been surprisingly found in those multi-layered filter pads that the layer assembly is kept intact during the filtration even in the filter packing that no longer contains binders. The filter pads or the filter packings produced therefrom according to the invention can, when used, and after reflushing and regeneration operations, finally be utilized, after being replaced, as filter aids in the deposit filtration process.

This is a continuation of co-pending application Ser. No. 06/915,227filed on Oct. 3, 1986, now abandoned which is a Divisional of U.S. Pat.No. 696,454 filed Jan. 31, 1985, now U.S. Pat. No. 4,647,415.

The invention concerns a process for producing a filter pad, which bymeans of binders is inherently stable and by removal of the binders canbe converted to a filter packing held together under substantiallyconstant surface pressure between rigid preferably platelike, filterbrackets for filtration and/or physicochemical treatment of liquidand/or gaseous media. The invention further concerns a filter padproduced according to said process and the employment thereof forfiltering and physicochemically treating liquid and/or gaseous media.

DE-OS No. 32 34 560 has disclosed a filtering device specially forbeverages wherein filter packings are held between rigid filter bracketsunder practically constant surface pressure. Said filter packings areinserted, for instance, as frustconical stable filter pads between thefilter brackets. As soon as a desired surface pressure that acts uponthe filter pad is produced with the filter brackets, the filter pads areconverted into granular filter packings by eliminating the binder. Buthitherto it has not been possible to predetermine to the desired extentand with the required reproducibility the utility, excellence andqualities of said filter packings.

The invention provides, in part an improved manufacturing process forfilter pads that can be converted to filter packings in which the filterpackings formed from the filter pads thus produced have filteringqualities that can be determined and selected in advance in combinationwith an increased total throughput capacity and corresponding increasedtotal service life, the same as improved adaptability during thefiltration, possibility of adjustment to a desired initial pressure dropand substantially improved insensitivity to pressure and thrust. Inanother aspect of the present invention the filter pads produced by thisnovel process are suited to improved methods of use.

Accordingly, the process of the present invention requires:

producing a suspension from one or more granular, fibrous orfiber-containing filter aids provided for the filter pad and/or one ormore mixtures of said filter aids with liquid,

forming in a casting mold a molded body from said suspension orsuspensions in the depositing cycle at predetermined rates of flow onthe order of at least 5 HL/m² h while avoiding turbulences, and

adding the binder to the filter aid or filter aids and/or the suspensionor suspensions, and stabilizing the molded body by activating the binderwhile drying.

With this novel process of manufacture it is possible to produce aninherently stable filter pad that can be safely transported and easilyand safely introduced in a filtering apparatus wherein it can beconverted to a highly effective filter packing of the desiredproperties. The filter packing formed from such filter pads stands outbecause of a specially high load capacity of the filter surfaces HL/m² hcapacity) and high total throughput capacity together with improvedregenerating capacity in closed state and high adaptability duringfiltration. The manufacturing process according to the invention can beeasily and reliably carried out and causes no environmental damage,since all the materials that take part in the process--as long as theyare not introduced in the filter pad--can be passed into the cycle.

The binder provided for producing the filter pad can preferably bedissolved and/or dispersed in the liquid of the suspension orsuspensions. This is possible because the liquid that remains in thefilter pad during the deposit contains sufficient binders to convert themolded body into the filter pad by drying and stabilizing.

Preferred binder materials include glucose, dextrines, gelatines,certain salts, silicates and the like and/or mixtures of two or more ofthe foregoing materials.

The filter pad can preferably be formed by forming in a flowable mold amolded body from the suspension or suspensions and converting saidmolded body in the mold into the filter pad.

In the process according to the invention in addition to the improvedfilter pads of the type mentioned above composed of a single layer, withthe process according to the invention the molded body can be formed byperiodically depositing consecutive layers that differ from each otherwith regard to their filtration properties, while using a suspensionthat differs with regard to particle size, and/or kind of filtrationaid, and/or composition of mixtures of filter aids. With this layerconstruction it is possible to produce filter pads that havepredetermined special properties. At the same time said specialproperties can be reproducibly controlled by selecting the respectivefilter aids and/or selecting the process parameters in the manufacture.Such a process parameter is, for instance, the rates of flow used in thedeposit operation with the selection of which it is possible toinfluence to a considerable extent the properties of the filter pad andultimately the properties of the filter packings formed from the filterpad.

In the process according to the invention it is possible to use in thedeposit cycle rates of flow of up to 500 HL/m² h and more, but in anycase turbulences in the deposit cycle should be avoided. Preferred ratesof flow for the deposit cycle can be between 100 and 300 HL/m² h.

Even though not necessary for the process according to the invention, itis possible to deposit in the mold the layers for the filter pads in adirection opposite to the provided direction of filtration of the filterpacking. The consecutively deposited layers can then be graduallycomposed of coarse filter aids that continuously become finer and at thesame time higher rates of flow can be used for forming the coarserlayers In addition, the deposit of the fine layers on the coarser layersoffers the advantage that in the depositing process the material of thefiner layers penetrates somewhat into the pores on the surface of thecoarser layers and thereby a substantially improved surface bond can beobtained between the consecutive layers.

In assembling a filter pad by depositing filter aids in a flowable mold,there is provided in the first place a molded body that must becomesolidified and stabilized by expelling the liquid. The liquid can beeliminated substantially by drying under elevated temperature. However,a fresh molded body produced by depositing filter aids will still lackthe required initial strength to be removed from the mold and introducedin a drier furnace. Accordingly, in a preferred embodiment of theinvention the molded body deposited can first be dried in the mold byflowing gaseous medium, preferably heated. This drying by blowinggaseous medium can be carried out as a pre-drying operation until themolded body in the casting mold has the sufficient strength for beingremoved from the mold and introduced in the drier furnace. But it isalso possible-e to carry out this drying by blowing gaseous medium in amanner such that the molded body is converted into the stable filterpad.

The stabilized dried filter pad can be provided on the outlet side ofthe filtrate, preferably also on the inlet side of the unfilteredsubstance, with a layer pervious to liquid and gas, but impervious toparticles of filter aid and resistant in respect to the filtrate. Suchlayers that are impervious to particles of filter aid can be produced,for example, by sealing the outer surface or surfaces of the filter padwith a suitable synthetic material such as a synthetic material having amelamine resine base. Finally, it is also possible in the processaccording to the invention to provide the inherently stable dried filterpad, at least on its edges, with border for protection against crumblingor damage. It is preferred firmly to sheath the filter pad all aroundwith a porous casing, preferably of tissue, in order to obtain optimalprotection on the edges and on the whole surface.

In the process according to the invention, it is possible to producefilter pads of practically any desired kind and with any desiredproperties. According to the process of the invention, it is possible inparticular to produce a filter pad characterized by being provided witha multiplicity of layers of different filtration properties that aremade from filter aids of different grain, and/or different textures,and/or different kind, and/or different mixtures, and/or differentproportions of ingredients, the adjacent layers having firm mutualsurface bond. If such a filter pad according to the invention isconverted into a filter packing and used for filtering liquid and/orgaseous media while maintaining a practically constant predeterminedsurface pressure, it is possible to carry out a multi-step filtration ina single filter packing. Here it has proved surprising that both duringthe conversion of the inherently stable filter pad into the state of thefilter packing and also during the reflushing and regeneration of suchfilter packings, the local arrangement of filter aids of different grainor other different properties is not lost and can be practically fullymaintained. Accordingly, filter pads made in accordance with theinvention also can be used in extended treatment processes for liquidand gaseous media such as bleaching treatment, deodorization treatment,enzymatic treatment, fermentation treatment, and others, and also in thecombination of such treatments one with the other and in combination ofsuch treatments with fine mesh filtration.

The filter pads and accordingly also the filter packing formed therefromcan have layers formed predominantly or exclusively from granular filteraids. Likewise the filter pad according to the invention and thereforealso the filter packing formed therefrom also can have those layerspredominantly or exclusively formed from fibrous filter aids of organicor inorganic kind.

The wide range of possible mixtures of filter aids makes it possible topredetermine the construction of a suitable filter pad for each productand for each filtration problem and accordingly to form from said filterpad an optimally suitable filter packing. In a preferred embodiment, thefilter pad can be, for instance, assembled in a manner such that--seenin the direction of filtration--the consecutive layers consist of filteraids that from layer to layer filter more finely. With a filter packingformed from such a filter pad, it is possible to achieve a very finefiltration culminating at a reliable disinfecting filtering action,there being guaranteed in addition to the assured filtration result, arelatively great absorption volume of impurities.

The layers of the filter pad can contain filter aids of all kinds suchas kieselguhr, (diatomaceous earth), perlite, bentonite, siliceous gel,fuller's earth, activated charcoal, celluloses, linters of cotton, orother suitable fibrous materials, alumimum silicate, polyamide,polyvinylpolypyrrolidone, and other synthetic materials. Byincorporating or employing other absorbing substances or absorbingsubstances, for instance, it is also possible to obtain bleaching and/ordeodorization as accessory or primary effects. The possible uses of thefilter pad or of the filter packings to be formed therefrom according tothe invention are so extensive that enzymatic and/or fermentativeprocesses can also be carried out thereby.

With a suitably assembled filter pad or filter packing formed therefromaccording to the invention, it is also possible to effect all kinds ofgas filtration.

The invention covers also the use of a filter pad produced by theprocess according to the invention for filtration and/or physicochemicaltreatment of liquid and/or gaseous media. In this connection it isproposed according to the invention that prior to a filtration operationthe filter pad be introduced between rigid, for instance, platelikefilter brackets, that for converting the filter pad into the filterpacking the binder be removed from the filter pad under a predeterminedcontact pressure on the filter brackets, and that the contact pressureon the filter brackets be maintained during the course of the filtrationand of all other process cycles above a minimum, preferablypredetermined, value. By the selection of the contact pressure it ispossible to adjust an initial value for the pressure drop, the filterpacking is already optimally effective at the beginning of theoperation. During the course of filtration, it is possible to monitorthe pressure drop that sets in the stream of unfiltered and filteredsubstances over the filter packing depending on the duration of thefiltration, to interrupt the filtration upon increase of the pressuredrop above a predetermined value and to wash out by reflushing thefilter packing that remains in its layer between the filter brackets, orwhen the pressure drop increases very quickly or the effectiveness ofthe flushing operation declines, to interrupt the filtration andregenerate the filter packing using acceptable chemical substances. Itis also possible to reflush and regenerate at predetermined intervals oftime.

As termination of a filtering operation, the same as termination of aflushing, cleaning or regeneration operation, it is possible with thefilter packing formed according to the invention from a filter pad toreduce to a negligible minimum the filtrate or the flushing, cleaning orregenerating agents, by squeezing out the filter packing by acceleratingthe contact pressure and subsequently blowing with an inert gas. In thismanner it is possible to reduce the loss of filtrate to a fraction incomparison with the known filtration process. By the complete removalfrom the filter packing, it is possible to collect completely thesubstances used for reflushing, regenerating and sterilizing the filterpacking and thus any environmental damage also can be prevented duringthe process steps.

Finally, when using the filter pad produced according to the invention,the consumed filter packing removed from between the filter brackets canbe used in devices for deposit filtration in order to utilize itscontents of filter aids as agents for deposit filtration, preferably inprefiltrations.

An embodiment of the invention is explained in detail herein below withreference to the drawing. In the drawing:

FIG. 1 is a schematic diagram of the process according to the invention;

FIG. 2 is a filter pad according to the invention shown in perspective;

FIG. 3 shows enlarged diagrammatic partial sections according to line3--3 of

FIG. 2 for a filter pad with:

(a) layers of fibrous filter aids,

(b) layers of granular filter aids, and

(c) layers of mixtures of fibrous and granular filter aids;

FIG. 4 shows in axial section a filter unit with filter packings formedfrom filter pads according to the invention;

FIG. 5 is a partial illustration of filter brackets and filter packingsin a unit according to FIG. 4; and

FIG. 6 is a plot of the pressure drop and of the performance in a filterpacking formed from a filter pad according to the invention withperiodically intervening cleaning and regeneration.

Referring to FIG. 1, there are provided supplies of material 11a, 11b,11c . . . 11g for different filter aids or mixtures of filter aids. Eachone of said supplies of material 11a, 11b, 11c . . . 11g is connectedvia a reciprocating pump 12a, 12b, 12c . . . 12g to be actuated undercontrol means (not shown) to a common material-collecting pipe 13 whichis connected via a centrifugal pump 14 controllable by its conveyingcapacity to a tank 15 for suspension liquid. The material-collectingpipe 13 leads to a mixing device such as the mixing cage 16 from which afeed pipe 17 for the suspension formed leads to the casting mold 18. Thecasting mold 18 is conically built in the example illustrated and has atits tip an inlet space 19 for the suspension. From there a mold cavity22 in which the molded body 20 is formed extends beneath the castingmold lid 21. The mold cavity 22 is downwardly confined by a depositplate 23 on the edge of which is situated an annular collector channel.To this annular collector channel 24 is attached a liquid outlet pipe 25that leads back into the tank 15 for the suspension liquid.

The supplies of material 11a, 11b, 11c . . . 11g are likewiseconstructed like a tank in which different filter aids or mixtures offilter aids are contained. These filter aids previously mixed withsuspension liquid can there be deposited dry or preferably in pastyform. The suspension liquid preferably has admixed therein apredetermined amount of binders. When the binder is dissolved in thesuspension liquid, a subsequent addition of binder to the suspensionliquid is not needed. It is only necessary from time to time toreplenish the supply of suspension liquid with binders dissolvedtherein, since the molded body formed in the casting mold 18 takesalong, when removed from the casting mold 18, a certain amount ofbinders contained in the suspension liquid.

In operation the circulation of the suspension liquid through thecasting mold 18 is maintained at the rate of flow desired via rotarypump 14. At the same time, from one or more of the supplies 11a, 11b,11c . . . 11g material in the desired amount is introduced in thematerial-collecting pipe 13, and thus in the suspension liquid passedinto the cycle, by actuating the respective reciprocating pump 12a, 12b,12c . . . 12g. An intensive blending then takes place in the mixing cage16 so that there is present in the feed pipe 17 a thoroughly blendedsuspension of filter aids. Said suspension is distributed in the moldcavity 22, the filter aids contained in the suspension being depositedon the deposit plate 23 or a layer of filter aids already formed on thedeposit plate 23. The suspension liquid practically free from filteraids then reaches, through the collector annular channel 24, the returnpipe 25 and from there the tank 15. If the binder is not dissolved butdispersed in the suspension liquid, it is possible, during the depositoperation that takes place in the casting mold, to obtain also aretension of binders in the deposited layers. In that case, eitherbinders are steadily poured in a predetermined amount in the tank 15 oran additional material supply for binders is connected to thematerial-collecting pipe 13.

For assembling molded bodies or filter pads having several differentlayers it is possible in a system according to FIG. 1 to introduce intothe material-collecting pipe 13, in consecutive periods of time and fromdifferent supplies of material 11a, 11b, 11c . . . 11g by thereciprocating pumps 12, 12, 12c . . . 12g, different filter aids overdifferent periods of time.

When a one-layered or multi-layered molded body 20 has been formed inthe above described manner in the mold cavity 22 of the casting mold 18,the lid 21 of the casting mold 18 is lifted and the resulting moldedbody 20 taken out to be dried and sterilized in a drier furnace underthe action of heat. Should it be necessary previously to solidify themolded body 20 to impart to it sufficient structural strength for beingtaken out from the casting mold 18, this can be done by blowingcompressed air which may be pre-heated, if desired. The molded body 20can be taken out from the casting mold 18 by suitable means that can beprovided, for instance, on the mold lid 21, or separately.

In FIGS. 2 and 3 can be seen examples of the assembly of a filter pad 10according to the invention. This filter pad 10 is made of filter aidsand in the example illustrated is frustoconical in shape. The filter pad10 is circumferentially surrounded by a porous casing 31. Said porouscasing 31 in the illustrated example consists of tissue, as it has beenhitherto known for use as filter cloth, and has a sealing edge 32 placedoutwardly beyond the outer peripheral margin of the filter pad 10, saidsealing edge 32 being secured with a burred closure or in any otheradequate manner to the part of the casing that is on the external sideof the filter pad 10. This porous casing 31 is mounted on the filter pad10 after stabilizing and drying and eventually surface sealing it.

In the examples of FIG. 3 the filter pad 10 has a multiplicity oflayers. In the example of FIG. 3a there are provided in the filter pad10 nine layers a to i made of fibrous filter aids. The layers a to gare--in the direction of filtration flow indicated by the arrow33--consecutively more finely made starting from a very coarse layer athat substantially acts as distributor for the unfiltered substance, andprogressing to a very fine layer g which filters sharply and sterilizes.The very fine layer g is followed--seen in the direction of thefiltration flow 33--by a layer h of medium coaseness that cancorrespond, for instance, to the layer b or c. Said layer h of mediumcoarseness is followed by a very coarse layer i that acts as collectorfor the filtrate on the filtrate exit side of the filter packing formedfrom the filter pad 10. The layer h of medium coarseness substantiallyacts at the same time as division between the very coarse layer i andthe very fine layer g.

In the example of FIG. 3b is given an analogous assembly of layers i tor made of granular filter aids. To this assembly logically apply thesame viewpoints explained above in connection with the example of FIG.3a. Unlike the example in FIG. 3a, there follows in the direction offiltration flow 33, after the very fine layer o, a layer p of mediumfineness that could correspond, for instance, to the layer m. This layerp of medium fineness is followed--seen in the direction of filtrationflow--by a layer o of medium coarseness similar, for instance, to thelayer 1 or the layer k. This is then followed--seen in the direction offiltration flow 33--by the very coarse layer r that can correspond, forinstance, to the layer j.

In the example of FIG. 3c the layers s to z are made of mixtures ofgranular and fibrous filter aids. For the rest, the same viewpointsexplained above in connection with the example of FIG. 3a do apply here.Of course, in the example of FIG. 3c the very fine layer x is alsofollowed--seen in the direction of filtration flow 33--by a quiteparticularly fine layer x' that can be suitable, for example, for highlyeffective sterilizing filtration.

The examples shown in FIG. 3 show only basic possibilities. There can beprovided, for example, a mixed assembly with layers made of purelygranular filter aids and layers made of mixtures of fibrous and granularfilter aids such as a layer assembly of--seen in the direction offiltration flow 33--j, k, c, d, w, x, x', y, r, or the like.

As example of the use of a filter pad according to FIGS. 2 and 3, thereis reproduced in FIGS. 4 and 5 a filter unit 40 wherein the filter pad10 is to be introduced together with its casing 31 between rigid filterbrackets 41. Said filter brackets 41 are shaped in the illustratedexample as conical annular plates that in the superposed sheets forminterstices. A filter pad 10 according to FIGS. 2 and 3 fits into eachinterstice between two adjacent filter brackets 41, the porous casing 31resting on the surfaces of the adjacent filter brackets 41 andsurrounding the peripheral edges of the filter packings formed. Theright part of FIG. 5 shows a number of superposed filter brackets 41with filter pads 10 inserted therebetween. Each filter bracket 41supports on the upper side a rack 43 for the filter pad 10, which rack43 is constructed with guide channels 42, and on the underside a rack 45made with guide channels 44. The stack of filter brackets 41 and filterpads 10 is situated around a central pull rod 46 that carries on itsupper portion an energy transmission body 47 that rests on a topmostfilter bracket 48. The pull rod 46 is connected at its lower end with ahydraulic cylinder-piston aggregate 49 with which it is possible toexert a controlled traction on the pull rod 46. For connection with afiltrate exit 50, each filter bracket 41 is provided on the outerperiphery with one or more connecting pieces 51. These connecting pieces51 communicate with the channels 44 of the rack 45. Flexible connectingpipes or hoses 52 are led from said connecting pieces 51 to thecorresponding terminals 53 on the tubular filtrate exit 50. The inletfor unfiltered substance 54 is centrally mounted on the boiler 55 thataccommodates the stack of filter brackets 41 and filter pads 10. At thesame time the channels 42 in the rack 43 of each filter bracket 41communicate with the interior space 56 of the boiler.

After introducing the filter pads 10 between the filter brackets 41 andclosing the boiler 55, the activation of the filter pads 10 has to takeplace prior to the beginning of the filtration. For this purpose theboiler 55 is filled and flushed through with water, preferably hot wateror a mixture of wet steam and water. There is simultaneously exertedwith the cylinder-piston aggregate 49 a predetermined constant tractionon the pull rod 46. During this activation process the binder isreleased from the filter pad 10. For this purpose it is convenient topress liquid such as hot water through the filter pad 10. This can beoptionally done in the direction of the filtration flow or opposite tothe direction of the filtration flow. By this activation the filter pads10 are converted into the filter packing 60 shown on the left part ofFIG. 5. A compression of the stack of filter brackets 51 and filter pads10 can here occur to a certain extent. But in said compression thearrangement and cohesion of the layers a to i, or j to r, or s to z ofthe filter pad are fully maintained. The filter pad 10 slightly deformsin the free spaces provided in the filter brackets, specially in thearea of the sealing rings 61 and 62 to be preferably used, in order toform a reliable sealing between the channels 42 of the rack 43 and thechannels 44 of the rack 45. Thereby all the product fed to the filterunit 40 is compelled to penetrate one of the filter packings 60 to reachfrom the interior space 56 of the boiler 55 the filtrate exit 50. Thesealing is permanently ensured by the fact that with the piston-cylinderaggregate 49 a constant pressure is maintained upon the pull rod 56 andthus a constant contact pressure of the filter packing 60 on the filterbrackets 41.

As FIG. 6 shows, there sets in during the filtration, under a constantfiltering capacity of about 10 HL/m² h, a periodically increasingpressure drop of, for example, 1.0 bar to 2.3 bar on the filter packings60. This pressure drop the same as the filtering capacity are monitoredduring the filtration. If upon reaching the maximum pressure dropadmissible at the moment of, for example, 2.3 bar, the filteringcapacity starts to decline, the filtration is interrupted. The filterpackings are then reflushed with flushing liquid and sterilized againwhile maintaining the contact pressure on the filter brackets 41. Thefiltration is then resumed, there appearing then again the course of thefiltration cycle a pressure drop on the filter packings 60 while thefiltration capacity remains constant. After repeated filtration cyclesand intervening reflushing, the effectiveness of the reflushinggenerally declines, which is evidenced by the more rapid occurrence ofthe pressure drop, that is shortening of the filtration cycles betweenthe reflushing operations. The filter packings can then be regenerated.The filter packings 60 remain between the filter brackets 41 also duringsaid regeneration, and the traction on the pull rod 46 and therewith thecontact pressure of the filter brackets 41 are also maintained at, oradjusted to, a value suited to the regeneration of the filter packings60. The regeneration takes place in the manner of a reflush, but notwith filtrate but with selected liquid chemical regeneration agents thatare suitable to dissolve organic residues (impurities), specially eggwhites and polyphenols, from the filter packings 60. By saidregeneration agents, there is even effected an intensive cleansing ofthe porous casing 31 and of the filter aids in the filter packings 60themselves. This preferably should be followed by a subsequent treatmentwith an acid solution with which inorganic impurities are also removed.

If it is desired to omit the constant monitoring of the pressure drop onthe filter packings 60, it is also possible to effect the reflush andregeneration operations at intervals of time to be predetermined withreference to empirical values.

The reflush and regeneration steps can be extensively improved on thefilter packings 60 formed from the filter pads 10 and maintained underconstant contact pressure by eliminating at the end of a filtrationcycle or flushing cycle, cleansing cycle or regeneration cycle therespective filtrate or flushing, cleansing or regenerating agent leavingonly a negligible minimum. For this purpose it is possible to increasethe contact pressure on the filter packings 60 to a considerablyelevated value without damaging the filter packing even in case of amulti-layered filter packing. The filter packings 60 thus considerablycompressed yield at the termination of the filtration the major part ofthe filtrate or rinsing, cleansing or regenerating agents retained. Thelast residues, except for a negligible minimum, also can be removed byblowing with an inert gas on the filter packing. This method isparticularly recommended when specially valuable products are to befiltered, as for instance in the preparation of brandy and the like. Inaddition, this method is recommended when one or more agents used inreflushing, regenerating, rinsing, acid post-treatment and sterilizationhave to be collected as completely as possible to avoid environmentaldamages.

If the filter packing is consumed after repeated regenerations andfrequent reflushing, the filter stack formed by the filter brackets 21and the filter packings 60 is disassembled and equipped with new filterpads 10, which are then activated as described above prior to thefiltration and converted to filter packings. Whether the filter packings60 have been consumed and have to be replaced, is determined either onthe basis of the constantly monitored pressure drop or on the basis ofempirical values with reference to the duration of the operation or thenumber of reflushing and regeneration operations. Prior to removing thefilter stack from the filter unit 40, one more regeneration operationsis effected on the filter packings 60.

The consumed filter packings typically still have, despite the removalof the binder, a solid inner content generally sufficient to permit eachfilter packing to be taken out intact from its porous casing 31 afterdrying. The porous casing, after adequate cleansing, may be used tohouse a new filter pad 10, while the consumed filter packing, which isstill held together as a relatively solid body, can be used aftersterilization as filter aid in the deposit filtration process,particularly for prefiltration purposes.

The conical shape of the filter pad 10 shown in the example explainedabove is given only as an example. It is possible adequately to producefilter pads of any desired shape such as square or circular plates ofthe desired dimensions. It is likewise possible to use the filter pad 10and the filter packings 60 formed therefrom in filter units other thanthe one of FIG. 4. Still other changes will be obvious to one skilled inthe art. Accordingly, it is intended that all matter contained in theforegoing description or shown in the accompanying drawings shall beinterpreted in an illustrative and not in a limiting sense.

I claim:
 1. A filter pad comprising a multiplicity of adjacent layers (ato i; j to r; s to z) of different filtration properties ranging fromcoarse to progressively finer filter layers, wherein the course layer isa base layer and the progressively finer layers are successivelydeposited thereon whereby each progressively finer layer at leastpartially penetrates into pores of the immediately adjacent depositedlayer thereby improving the surface bonding between adjacent layers, andsaid filter pad being rendered dimensionally stable by means of a bindermaterial and being convertible to a filter packing by an in-situ removalof said binder material.further comprising at least one layer thatcontains agents which enzymatically treat the medium to be filtered. 2.A filter pad according to claim 1, wherein at least one of said adjacentlayers (j or r) is predominantly formed from a granular filter aid.
 3. Afilter pad according to claim 1, wherein at least one of said adjacentlayers (a to i) is predominantly formed from a fibrous filter aid.
 4. Afilter pad according to claim 3, wherein said filter aid comprises anorganic material.
 5. A filter pad according to claim 3, wherein saidfilter aid comprises an inorganic material.
 6. A filter pad according toclaim 1, and comprising at least one layer of an absorbing agent.
 7. Afilter pad according to claim 6, wherein said absorbing agent isselected from the group consisting of kieselguhr, perlite, bentonite,fuller's earth, silicious gel, activated charcoal, cellulose,polyvinylpolypyrrolidone, linter of cotton, aluminum silicate andpolyamide.
 8. A filter pad according to claim 1, wherein at least one ofsaid adjacent layers (a to i) is exclusively formed from a fibrousfilter aid.
 9. A filter pad according to claim 8, wherein said filteraid comprises an organic material.
 10. A filter pad according to claim8, wherein said filter aid comprises inorganic material.
 11. A filterpad according to claim 1, and comprising at least one layer of anabsorbing agent.
 12. A filter pad comprising a multiplicity of adjacentlayers (a to i; j to r; s to z) of different filtration propertiesranging from coarse to progressively finer filter layers, wherein thecourse layer is a base layer and the progressively finer layers aresuccessively deposited thereon whereby each progressively finer layer atleast partially penetrates into pores of the immediately adjacentdeposited layer thereby improving the surface bonding between adjacentlayers, and said filter pad being rendered dimensionally stable by meansof a binder material and being convertible to a filter packing anin-situ removal of said binder material,comprising at least one layerthat contains agents which fermentatively treat the medium to befiltered.